A method for fault location must necessarily be preceded by some form of detection of the fault. A detection can take place in many different ways which will not be described here, with the exception of the special method of detection which is a condition for the method and the device for fault location according to this invention. This detection will be described in connection with the disclosure of the present invention.
Concerning fault location on a transmission line, a plurality of different known methods exist.
One such method is known from British patent application No. 2,036,478A. This describes a method by which a location for a fault is assumed and, under simplified assumptions of the parameters of the network, the current and voltage at the assumed fault location are determined. The determinations are repeated at different assumed locations until the determined current and the voltage have the same phase, which means that the fault point has been correctly located.
Another method is described in U.S. Pat. No. 4,559,491 (Saha). This publication relates to a method for fault location on a section of a three-phase power line. After measurement of currents and voltages at one end of the section, the type of fault is first determined and thereafter certain parameters in an equation system are determined. The equation system used provides relationships between the complex values of the impedance of the section, the impedances of the networks of the near end and of the remote end of the section, and measured currents and voltages while eliminating fault resistance, zero sequence components, etc. Solving the equation system gives the distance from the end point of the section to the location of the fault in question.
In an invention described in U.S. application Ser. No. 743,930, filed on June 12th 1985 now U.S. Pat. No. 4,719,580 in the name of Nimmersjo, a method and a device for locating a fault are described, which is based on voltage waves emanating from a measuring point towards the fault location and corresponding waves reflected from the fault location, which waves are included in a travelling wave model of the transmission line. The invention described in this earlier application comprises carrying out, at certain specified time intervals, a measurement of the instantaneous values of the current and voltage at the end point of the transmission line, for example in a station. With these measured values and with the aid of the travelling wave model, the voltage at a number of control points along the transmission line can be calculated. If the transmission line is energized and the calculated control voltages at two consecutive control points constantly have different signs, there is a fault on the transmission line between these points. Starting from the distance between the points in question and the calculated control voltages, the location of the fault can be determined by interpolation.
It is the travelling wave model described in the above-noted prior application that forms the basis for the fault location method and equipment according to the present invention which is hereby incorporated by reference.